Solid-state digital X-ray detectors, also referred to as X-ray sensors, can be constructed by employing either of two physical detection methods, so-called direct and indirect conversion methods. Generally, a direct conversion method makes use of direct production of electrons by X-rays in elemental compounds such as amorphous silicon or selenium, lead oxide, lead iodide, thallium bromide, or various gadolinium compounds. In this case, the electrons are collected via electric fields and electrodes attached to thin film transistors. On the other hand, an indirect conversion method employs conversion of X-ray interactions to flashes of light in well-known scintillating materials such as thallium-activated cesium iodide or gadolinium oxysulfate. In this case, the light flashes are sensed by photodiodes, and the resulting electron currents are again collected by attached transistor electronics.
The collecting electrodes or photodiodes are configured as a pixel field embedded in a large flat panel configuration, and images are formed in a manner similar to conventional digital cameras and video displays through electronic processing of the collected charges. Once formed, these images can be displayed on video monitors, printed on film or paper, or placed in an electronic storage system for later retrieval.
Both direct and indirect detectors are typically made of thin-film-transistor (TFT) arrays for pixel processing and readout. These TFT arrays offer a variety of advantages over traditional X-ray imaging systems. Compared to conventional screen film systems, a film-less system can be achieved, facilitating improved image quality based on digital image processing, diagnostic support, electronic filing, and networking. However, limitations of TFT arrays include resolution, contrast and noise. These three parameters describe the image. There are three other intermediate parameters that tie in these three parameters, including spatial frequency, modular transfer function (MTF) and signal-to-noise (S/N) ratio. The spatial frequency or spatial resolution of an imaging system can be defined in terms of smallest spacing between two objects that can be imaged clearly. It is measured in terms of line pairs per millimeter or lp/mm. The MTF describes the contrast produced by an imaging system as a function of spatial frequency of an object. Another concept in determining the quality of imaging system is detective quantum efficiency (DQE). DQE is the ratio of S/N at the source to S/N at the signal output. DQE influences the X-ray dose required to create a good quality image.
S/N ratio, MTF and DQE are used to determine how good an X-ray detector is in converting X-rays through objects into a good quality image. S/N and MTF are very dependent of two structural parameters of X-ray detector, namely: pixel pitch and noise generated by the sensor due to interaction of X-ray with sensor material and electronic noise generated by electronics.
FIG. 1 shows a typical indirect TFT X-ray imaging detector that has a layer of scintillator 13 to convert X-ray 11 into light that is placed on the top of an active pixel array circuit 16 made in an amorphous silicon substrate 14. The active pixel 15, 17 has a photodiode 18 to detect light and transistor(s) 19 to amplify the signal from the photodiode. Sometimes three or more transistors and a diode are used in an active pixel to reduce the noise. Detected light is converted in to electrical signal by the photodiode 18. Signals from the photodiodes are sensed by a transistor 19. Select lines 111 and sense lines 110 are connected to the transistor to bring out the sensed analog signal to the edge of the detector die. Then the analog signals are converted to digital signals, amplified and encoded and streamed out to perform next level of signal processing to create an image 115 of the sensed object 12 displayed in monitor 114 by connecting sensor substrate 14 to computer 113 through wire 112.
The resolution of an X-ray detector is limited by either size of the active pixel or scintillator resolution, whichever is lower. Pixel dependent resolution can be calculate by 2× width of the active pixel. Scintillator resolution determined by the scattering light which in turn depends on thickness of the scintillator. Resolution or lp/mm (line pair per millimeter) at a fixed MTF can be calculated by 2× thickness of the scintillator approximately. For example, if active pixels pitch is 200 microns, then active pixel limited of resolution will be 2.5 lp/mm and if scintillator thickness is 500 micron then scintillator limited resolution will be 1.0 lp/mm. Thus, the overall resolution of modern indirect TFT detector will be about 1.0 lp/mm in this example. Resolution is defined here as lp/mm at a fixed MTF.
A typical direct TFT X-ray detector has a layer of photoconductor on the top of an active pixel array. The active pixel has an electrode, a capacitor and transistors. Since photoconductor has significantly less scattering of electrons, the resolution of direct TFT is limited by the active pixel pitch which is about 100 to 200 microns, and should be 5.0 lp/mm at MTF of 50%. However, due to the noise in amorphous silicon and electronic noise, best resolution observed is less than 2 lp/mm.
The active pixel area in a TFT detector is occupied by a photodiode or an electrode with capacitor and transistors. The transistors in pixel occupy most of the pixel area because they are very large due to very low mobility of the amorphous silicon. Thus, the ratio of photodiode or electrode area and the total area of the pixel termed as fill factor determine what percentage of X-rays has been sensed. Both direct and indirect detectors have fill factor of less than 30%. Fill factor influences the dose required to create a clear image in a detector.
Recently TFT is being replaced by single crystal silicon based detectors to overcome the above limitations of TFT detectors. Those limitations are resolution and fill factor. These silicon based detectors have lower noise, higher dynamic range, smaller pixel capability and higher fill factor. Hence they have potential for high resolution, high DQE, and higher readout speeds. Silicon based detectors are designed for indirect conversion of X-rays into electron. The reason is that the silicon based detectors are driven by CMOS based optical camera technology. Thus theoretically, very high resolution is possible with silicon based detectors. Pixel pitches of as low as 2 microns are being manufactured by digital camera companies. High resolution and DQE of 70% was obtained. This was due to less than 100 micron active pixels with high fill factor, low noise and high dynamic range.
However, there are still several limitation in improving X-ray detector physical size, image quality and dose requirements even with current with silicon based detectors technology. These limitations are described below.
Size of a silicon detector panels is a big limitation today. Size of a TFT panels today is about 100 cm×100 cm which is needed for most medical applications. Largest silicon panel today is 20 cm×25 cm made with 4 or 6 dies tiled together. This tiling causes missing pixel rows and columns which affects quality of image. It is not possible to increase the silicon panel size due to two reasons.
First is the size of the die and second is the number of the tiles that can be tiled together. Size of the die is limited by the yield of dies on wafer due to defects.
Since pixels and select and sense circuits are on the same die, the die can be tiled only on three sides. The fourth side is used for select and sense circuits.
The smaller the pixel, the higher the resolution will be. Best pixel size in silicon large X-ray detector is about 100 microns giving a resolution of 5 lp/mm. This resolution corresponds to an object of the size of about 1 millimeter clearly in an image. To see anything smaller one would need higher resolution. However, any reduction in pixel size but still for a given die size results in lower yield of the die.
Die size is limited by number of transistors on a die. As an example, following table shows yield of a large die as a function active pixel size and size of detector panel for a given defect density. Largest size of die possible with 300 cm diameter silicon wafer is 20 cm×20 cm. Lithography assumed is 180 nm. Area of a typical transistor is 4 micron. Yield is calculated using industry standard method.
Pixel size (in microns)10010010050206Die size (in cm on51020202020side)No. of pixels on the0.251.004.00161001000die (in millions)No. of transistor per0.753.0012.00483003000die (millions)(3 transistor/pixel)Die Yields97%89%62%15%0%0%(for 1 defect/cm2)
Yield decreases as the die size increases and pixel size decreases. Loss of yield translates into cost of the detector die. It is not economical to build a large die that is 20 cm×20 cm with pixels size below 100 microns. In addition one could improve the image quality if more than three transistors can be used per pixel. It has been demonstrated that by adding functions such as A/D and memory, one can increase speed or frame rate significantly to make video images. These added functions will require adding more transistors per pixel. One of the limitations of the silicon die is that yield loss is exponential as one increases the number of transistor per pixel as shown in the table below.
No. of transistorper die361224Pixel size (in microns)100100100100Die size (in cm on side)20202020No. of pixels on the die (millions)4444Die Yields (for 1 defect/cm2)62%38%15%2%
Thus, even with 100 micron pixel size the die yield will drop quickly if transistors per pixel are increased beyond three transistors.
Best resolution today on silicon detector is about 5 lp/mm on 8 cm×15 cm die with pixel size of 100 microns. If yield problem can be solved resolution can go to more than 80 lp/mm.
The speed at which digitized image data can be obtained becomes very important parameter when the amount of data per frame increase exponentially due to reduction of size of pixel. A one-megapixel silicon detector with a pixel size of 200 microns generates 14 Mb of data that has to be extracted in one exposure time of one second or at the rate of 14 Mb/sec. A large silicon detector with small pixel may require 10 times to 100 times the data rate required in one megapixel detector. Data extraction from the die at those rates is not possible today due to the number of transistors available per pixel.
Electronic noise is an important component of the S/N (signal to noise ratio) at the output of the detector die. Amplified analog signals from photodiode in each pixel are carried to the edge of the detector die through set of sense lines. Cross talk on these sense lines is a source of noise that affects S/N at the output.
Most scintillators are films of materials that convert X-ray into light. The main limitation of these scintillator film are the amount of light spreading generated from each X-Ray photon sideways into neighboring pixel. The amount of light spreading limits the resolution of the film. Thus, even if silicon detector has high resolution, it may be limited by the scintillator. Typical films are 300 to 500 microns. Limiting resolution to 1.0 lp/mm. One could increase resolution by thinning the thickness of the scintillator to 100 microns but that means all the X-rays are not absorbed by the scintillator. Hence it will take more X-rays to get the same quality of image or higher dose. Unabsorbed X-rays will damage the electronics or the objected that is being exposed. To prevent these unabsorbed X-rays a fiber optic plate is place to absorb the X-rays as well as collimate the light it improve the resolution. Resolution has been improved to 5 lp/mm. Key problems with fiber optic plate are X-rays passing through space between fibers affecting quantum efficiency as well affecting electronics and object being X-rayed.
A grid structure as in FIG. 2 is used to reduce the spread of light. FIG. 2 shows a typical layout of a grid on a silicon die with active pixel containing photodiode. A grid is placed on the silicon die 21 with active pixels 23. The grid holes 22 are aligned to active pixel 23. The section AA′ in FIG. 2 is shown in FIG. 3. Grid 31 with holes 35 filled with scintillator material 32 such as CsI is placed on a substrate 33 with active pixels 34. In one approach, a grid is made with Tungsten substrate and filled with scintillator material. Grid holes act like optical waveguide limiting spreading of light to adjacent pixel. More recently, in another approach, silicon grids as that shown in FIG. 4 are used to create optical waveguide to limit light scattering to adjacent pixel. The silicon substrate 44 with active pixels 46 are placed below grid 41 with holes 43 filled with CsI. The oxide 42 in the sidewalls of hole makes light generated in the CsI to confine in the hole by total internal reflection. Limitation of grid scintillator has been the inability of precisely aligning grid holes to the photodiode as in FIG. 4. When grid 41 is placed on silicon die 44, pixel 46 is misaligned to the grid hole 43. Misalignment 45 causes significant crosstalk in adjacent pixels leading to noise. Difficulty in aligning is due to inability to see alignment marks on the silicon die clearly through the grid holes filled with scintillator material. This problem becomes more acute as pixel sizes are reduced to increase resolution. Presently this problem limits pixel size to about 100 microns.
Another type of silicon detector has also been developed in industry by integrating silicon dies in three dimensions (3D) or by stacking dies and connecting them with vertical connectors. In one 3D scheme photodiode array can be placed in a very large die (photodiode die) whose size is limited by the biggest silicon wafer available. Another die (CMOS die) of same size is designed with pixel addressing CMOS circuits. There are three limitations in this scheme as described below.
First, size of pixels is limited by the accuracy with which CMOS die can be aligned with photodiode die. Alignment limits the size of pixel to about 40 microns. Second, yield of the large CMOS die becomes very low as its size increase for the same reasons as discussed above in the case of active pixels. Third, scintillator limits the resolution as described in active pixel case previously.
A direct detector may also be utilized for imaging applications. The detector is constructed by placing CdTe die with Schottky diodes on the CMOS die. Operation of this direct detector is very similar to indirect 3D detector described earlier. There are, however, limitations to this type of detectors, as described below.
First, size of pixels is limited by the accuracy with which CMOS die can be aligned with photodiode die. Alignment limits the size of pixel to about 40 microns. Moreover, yield of the large CMOS die becomes very low as its size increase for the same reasons as discussed above in the case of active pixels.
In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustrating specific exemplary embodiments in which the disclosure may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the concepts disclosed herein, and it is to be understood that modifications to the various disclosed embodiments may be made, and other embodiments may be utilized, without departing from the scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense.